[S-GC32] Volatiles in the Earth - from Surface to Deep Mantle

[SGC32-P04] Fate of Subducting Organic Carbon: Evidence from HP/UHP Metasedimentary Suites and Implications for Isotopic Compositions of Arc Volcanic Gases

*Gray E Bebout¹, Kaylee H. Kraft¹, Jennie Pinho², Gabe S. Epstein¹ (1.Lehigh University, Dept. Earth and Environmental Sciences, Bethlehem, Pennsylvania, USA, 2.Lafayette College, Dept. Chemistry, Easton, Pennsylvania, USA)

Keywords:carbon cycling, subduction, metamorphism

Western Alps high-pressure and ultrahigh-pressure (HP/UHP) metasedimentary rocks record transit of organic (reduced) C through forearcs (40-70 km) to depths approaching those beneath volcanic fronts. The Schistes Lustrés, exposed in the Italian and French Alps, represents subduction of sediment to depths of up to ~70km, where they were underplated, along an extremely low-P/T trajectory of 7-8ºC/km. Across the range of grade in this suite (peak temperatures of ∼350-550ºC), reduced C shows little evidence of loss (based on normalization to concentrations of Al₂O₃, TiO₂, and Zr) but considerable increase in δ¹³C_VPDB to near mantle values of -6‰ related to isotopic exchange with coexisting carbonate. In Schistes Lustrés metashales lacking carbonate, reduced C preserves protolith δ¹³C of -24 to -21‰ independent of grade and degrees of devolatilization. Reduced C occurs as amorphous carbonaceous matter at low grades and its transformation to graphite at higher grades presumably enhances isotopic exchange with carbonate. For comparison, in carbonate-poor lower-grade units of the Catalina Schist (in California), and Franciscan Complex (California) and Western Baja Terrane (Mexico) blueschist-facies rocks, together representing subduction to 10-40 km depths, reduced C concentrations and δ¹³C, C/N, and δ¹⁵N are similar to those of sediment protoliths (see Sadofsky and Bebout, 2003; G-cubed), as in the lowest-grade unit of the Schistes Lustrés (regardless of carbonate content; for the N isotope data see Bebout et al., 2013; Chem. Geol.). In Catalina Schist units that experienced warmer prograde P-T paths, reduced C is shifted in δ¹³C from -25 to -19‰ related to minor loss of C as CH₄ during devolatilization.

Our work implies that much of the reduced C reservoir is retained in rocks subducting to depths approaching those beneath volcanic fronts and could be available for transfer into the sub-arc mantle wedge, perhaps via partial melting. Yet unknown is whether reduced and oxidized C reservoirs, and their isotopic compositions, are fractionated during sub-arc mobilization and delivery into the mantle wedge. Such fractionation, and any isotopic exchange between the two reservoirs (in addition to accretion and underplating; see House et al., 2019, Geology), could have implications for application of the CO₂/³He-δ¹³C approach used in studies of arc volcanic gases to calculate contributions from mantle and disparate sediment sources.